Coaxial microwave attenuator having conical radial line absorbing members

ABSTRACT

Coaxial microwave attenuator for use at high power, which is independent of the frequency being transmitted. The attenuator comprises a series of conical absorption members assembled to obtain the desired attenuation.

BACKGROUND OF THE INVENTION

This invention relates to a coaxial microwave attenuator for high power,which operates independently of the frequency and in particular onehaving a fixed or adjustable structure. Such attenuators are frequentlyused in high frequency and microwave techniques.

In wave guide systems there is a clear tendency to increase the powerstransmitted. Coaxial components such as attenuators or terminalresistors adapted to frequencies of more than 4 gigahertz and to powersgreater than 10 watts are difficult to find on the market.

Fixed attenuators of a conventional type for frequencies ranging up to18 gigahertz will only carry loads of a few watts. Such units are oftengrouped in the form of large cylindrical adjustable attenuators. If itis desired to obtain a graduation corresponding to variations of 1decibel such attenuators are expensive. The coaxial or flat resistors ofconventional attenuators are for the most part mounted on the innerconductor. This results in an undesirable transfer of heat to the moremassive parts of the external conductor of the attenuator, whichexplains its low maximum load. If attenuators having directionalcouplers are used, it is possible to increase the maximum load byplacing at the end of the direct line of the coupler a resistance havinga high loading capacity. When the line is coupled one may, however,accommodate a minimum attenuation of about 10 decibels, which is oftennot very desirable.

In French Pat. No. 70.06639, an attenuator is proposed in which most ofthe aforesaid deficiencies are avoided. In order to obtain over a largeband (for example 10:1 and more) an attenuation curve which isindependent of the frequencies, one is also obliged, in this solution,to combine various individual absorption members assembled in series,one after the other, into a complete attenuator.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view in section of a prior art coaxial microwaveattenuator;

FIG. 2 is a block diagram of characteristic impedances of the attenuatorof FIG. 1;

FIG. 3 shows another form of prior art attenuator;

FIG. 4 shows another form of prior art attenuator;

FIG. 5 shows a prior art disc attenuator;

FIG. 6 shows a conical attenuator in accordance with the presentinvention;

FIG. 7 shows a modified form of attenuator according to the invention;

FIG. 8 shows another embodiment of the attenuator according to theinvention, further having a heat dissipating fin; and

FIG. 9 shows an adjustable embodiment of an attenuator according to theinvention.

The present invention has the object of providing individual absorptionmembers which are identical to each other, function independently of thefrequency, and which may be assembled in series one after the other toobtain the desired degree of attenuation.

Such an attenuator is made, like the one in French Pat. No. 70.06639, bymounting in series a certain number of individual potentiometers. Inorder to explain the new principle of the attenuator operatingindependently of the frequency a single coaxial line connected in serieswill first be described.

In the following explanation, FIGS. 1-5 represent the prior art.

FIG. 1 shows a line of this type becoming a voltage divider when theelement of the coaxial line of low impedance constituted by the externalconductor 10 having a diameter D₁ and an inner conductor 12 having adiameter D₂ is provided with a non-reflective terminal resistance R, andwhen the element of the coaxial line constituted by conductors 12 and 14and having the diameters D₂ and d respectively, is terminated in itscharacteristic impedance. The characteristic impedances may bedetermined from FIG. 2 as follows:

    Z.sub.1 =(138/√ε.sub.r) Log (D.sub.1 /d) [Ω]

    Z.sub.2 =(138/√ε.sub.r) Log (D.sub.2 /d) [Ω]

    ΔZ=(138/√ε.sub.r) Log (D.sub.1 /(D.sub.1 -2b)) [Ω]

for b<<D₁,D_(1/2) is ≈D, and therefore Z₁ ≈Z₂ ≈Z

The transitional attenuation of the voltage divider may be evaluatedfrom the partial voltages in accordance with FIG. 2:

    u.sub.2 =u.sub.1 z/(z+Δz)

    p.sub.2 =u.sub.2.sup.2 /z=(u.sub.1 z/(z+Δz)).sup.2 /z=(u.sub.1.sup.2 ·z)/(z+Δz).sup.2

    p.sub.1 =u.sub.1.sup.2 /(z+Δz)

    (p.sub.1 /p.sub.2)=(u.sub.1.sup.2 /(z+Δz))·((z+Δz).sup.2 /(u.sub.1.sup.2 ·z))=(z+Δz)/z

    a[dB]=10 Log (P.sub.1 /P.sub.2)=10 Log (Z+ΔZ)/Z

this attenuation is independent of the frequency because it is afunction only of the geometric masses of the voltage divider. If theelongated hollow annulus between conductors 10 and 12 is filled with adielectric e subject to losses, up to the surface of the inner wall ofthe outer conductor, as shown on FIG. 3, the impedance ΔZ√ε_(r)evaluated from FIGS. 1 and 2 depends only on the relative dielectriccoefficient ε_(r) and the coefficient of permeability μ_(r) of thedielectric, when the length of the annulus l from the end y of theshort-circuited annulus is so large that for the lower limit of theselected frequency f.sub.μ no appreciable reaction is produced at thebeginning Z of the annulus. It follows that:

    ΔZ√ε.sub.r ≈138·K ·log (D+2·b/2)/(D-2·b/2)[ω]

from which:

    K≈═√(μ.sub.r /ε.sub.r)═, Z coax=138 log (D/d)[ω]

ε_(r) and μ_(r) are magnitudes which depend generally on the frequency.Measurements made on various compositions of materials have made itpossible to attain a favorable dielectric mixture comprising threeparts, one of which is non-magnetic, another of which is magnetic andanother part which is made of a moldable resin for instance an epoxyresin with a heat polymerizing catalyst. The coefficient K=√(μ_(r)/ε_(r)) of this mixture is to a large extent independent of thefrequency. The attenuation during transit is then:

    A.sub.[dB] =10 log (Z+ΔZε)/Z≈10 log (1+K·(log (D+b)-log (D-b))/(log D-log d))

that is to say, the attenuation is above the lower limit of frequencyf.sub.μ, independent of the constant of frequency and depends only ongeometry and K.

Practical attenuators cannot be made as shown in FIG. 4 because, byreason of the length necessary for the member q, an attenuatorcomprising several members would be much too long.

The member q can be mounted only perpendicularly to the direction of theline as shown in FIG. 5. Thus q forms a radial line subject to losseswith a spacing b between conductors. The radial lines having a constantspacing between conductors are not homogenous, that is to say theircharacteristic impedance decreases as one moves away from the center C.

This is also why the condition for holding the partial impedanceΔZ√ε_(r) constant independently of the attenuation frequency is notfulfilled.

The novelty of the present invention resides in the fact that there isutilized, as a radial attenuation line, not the one shown in FIG. 5, buta radial line having a conical shape as shown on FIG. 6. If the summitsof the cones of the surfaces delimiting this radial line coincide withthe center C of the coaxial line this line is homogenous; that is to sayits characteristic impedance is independent of location and frequency.For this reason ΔZ√ε_(r) and the attenuation α are constants independentof the frequency. The dimension 1 in FIG. 6 or in FIG. 7 must, for itspart, be selected large enough that when the frequency is f.sub.μ thereaction of the edge of the short-circuited metallic disc f as shown forexample in FIG. 7, remains negligibly small.

Considerations of manufacture have led to an absorption member such asshown on FIG. 7. It comprises an absorption member q and a conicalcomplementary metallic ring f. The two cemented parts form a flat disc Ehaving a central hole, for example, for 5 mm of its thickness, andprovides an attenuation of 0.5 decibels. The exterior surfaces g aremetallized to obtain definite conductive surfaces.

There is obtained, for example, by joining 20 similar discs to eachother the following electrical specifications:

Attenuation A: 10 decibels

Average uncertainty of attenuation:

    ΔA.sub.m =±0.2 decibels

Uncertainty of attenuation on the frequency curve:

    ΔA.sub.f =±0.2 decibels

Approximate range of frequency: 1.7-37 gigahertz (20:1)

Charging capacity (without auxiliary cooling surface): 50 watts of DCpower. p1 Reflection factor: r≦0.1

For example if one selects a coaxial line having an external conductorthe diameter D of which equals 3.5 mm while the internal diameter of theconductor d=1.51 mm a cutoff wave length

    λc=π((D+d)/2)=7.87 mm

is obtained, which results in a cutoff frequency Fc: 38 gigahertz.

The connectors actually available on the market have a cutoff frequencyof 37 gigahertz. The present invention may be used over very largefrequency ranges when connectors having a higher cutoff frequency becomeavailable.

FIG. 8 shows an advantageous embodiment of the attenuator according tothe invention in which a fin A made of aluminum or any other materialhaving a low thermal resistance is placed between two absorption membersq such as shown on FIG. 7. This fin A may have any geometric form butmust have a sufficient surface area to dissipate enough power tomaintain an acceptable temperature for the attenuator. Each fin A may,for example, permit the dissipation of 50 watts at a temperature below110° C.

An adjustable embodiment of the attenuator may, for example, be made bycutting slots S into the individual members E (as shown in FIG. 9),which slots extend tangentially outwardly from the internal surface ofthe outer conductor F formed by the members E. Thanks to a thin metalcontact sheet B in the form of a strip capable of being wound helicallyby external toothed wheels (not shown) around a cellular dielectric Di,all the absorptive members E are progressively, beginning at 0 decibels(when sheet B fully covers the cellular dielectric Di), successivelyseparated by a screen (the wound strip B) from the magnetic wavescirculating in the coaxial line. In the completely screened state, whenthe sheet B completely covers the dielectric Di which is within membersE the coaxial line consists of the strip B as an external conductor andI as an internal conductor. Sheet B can be unwound or wound through theslots S, the attenuation being 0 decibels when the sheet is fully wound,and increasing as the sheet is unwound to progressively expose more ofthe absorbent elements E.

The weakly absorbent cellular dielectric Di, having for example adielectric constant of approximately 1, is advantageously made from aplurality of tubes coated with a mixture of the three componentsdescribed, and threaded on the internal conductor I.

As a variation, the dielectric may be located in peripheral grooves inthe inner conductor (FIG. 10a) or in longitudinal slots therein (FIG.10b). The modified internal conductors I of FIG. 10a and FIG. 10b can beused as the internal conductors of any of the embodiments of FIGS. 6-9.

FIG. 11 shows the attenuation as a function of frequency. Curve 1represents the attenuation without the weakly absorbent dielectric(Embodiments of FIGS. 6, 7, 8); the curve 2, the attenuation providedwith this dielectric (Embodiments of FIGS. 6, 7, 8 with the innerconductor I of FIGS. 10a and 10b); the curve 3 the attenuation providedwith this dielectric after screening (embodiment of FIG. 9). It will beseen that, beginning at a predetermined frequency, the attenuationbecomes independent of the frequency.

The electrical specifications of this embodiment of adjustableattenuator are the same as those for the fixed embodiment except withrespect to the range of attenuation which is about 0-30 decibels.

In accordance with another embodiment (not illustrated) the attenuatorcomprises a device (not shown) for covering or uncovering an adjustablenumber of absorption discs made of a straight coaxial telescopic line ofthe trombone type.

The high maximum load afforded by the new method of construction resultsfrom the fact that, contrary to known attenuators, the attenuatinglayers are positioned in the outer conductor. The high resistance tothermal variation of the solid absorption discs makes it possible torelease the microwave energy withdrawn and transformed into heatdirectly into the atmosphere. An evaluation of the heat flow shows thatthe greatest thermal resistance appears at the transition point betweenthe external surface of the outer conductor and the ambient air. Fromthis point the absorption of power received by the attenuator dependsprincipally on the nature of the cooling surfaces and on the temperatureat which they have been received. Experiments with a prototype providedwith smooth metallic surfaces serving as a transition point with theambient air have produced an attenuator having an admissible loadingcapacity of 50 watts direct power for 10 decibels and 10 centimeters ofattenuator length and an external surface temperature ≦ than 100° C.(ambient temperature 20° C.). With a cooling body having optimumdimensions one may obtain for the same attenuation and the same lengthof construction an admissible load of at least 100 watts continuouspower.

What is claimed is:
 1. Wide-band coaxial microwave attenuator whichcomprises a coaxial line element provided at its ends with coaxialleads, having a smooth inner metallic conductor and an outer conductorcomposed of conical radial line members similar to each other andopening outwardly, said conical members being made of a material subjectto dielectric losses at a relative ratio which is constant as topermeability, constant dielectrically, and independent of frequency,said line members being grouped in a compact attenuator by complementarymetallic conical rings between the sides of said line members, and saidline members being adapted to be assembled to form a regular rowconnected in series to form a voltage divider having a selective valueof attenuation.
 2. Coaxial wide-band microwave attenuator as claimed inclaim 1 in which the dielectric used comprises three materials, one ofwhich is magnetic, another non-magnetic and the third a moldable resin,said attenuator also comprising a weakly absorbent dielectric whichassures that the attenuation curve which, without this dielectric, woulddecrease slightly as the frequency increases, is practically independentof the frequency.
 3. Attenuator as claimed in claim 2 in which theweakly absorbent dielectric is in the form of an elongated bodypositioned in the space between the inner conductor and the outerconductor.
 4. Attenuator as claimed in claim 2 in which the weaklyabsorbent dielectric is positioned in peripheral grooves in the innerconductor.
 5. Attenuator as claimed in claim 2 in which the weaklyabsorbent dielectric is positioned in longitudinal slots in the innerconductor.
 6. Adjustable wide-band microwave attenuator according toclaim 1 in which the line members comprise individual absorption discshaving a slot extending tangentially outward from the internal wall ofthe outer conductor, a contact sheet in the form of a strip passingthrough the slot and wound on a cellular body in the space between theinner and the outer conductors, said sheet screening said absorptiondiscs from the magnetic waves between the sheet and the inner conductorto provide zero attenuation when the sheet is fully wound, and unwindingsaid sheet through said slot progressively exposing said absorptiondiscs to the electromagnetic waves to increase attenuation.